The present invention relates to a method of the biological control of pollen in plants using RNase. The present invention particularly relates to the thinning of a large variety of fruit plants, the reduction of seeds in citrus plants and the reduction of the incidence of shotberries in grapes using activated RNase, inactivated RNase or mixtures of these.
The purpose of thinning fruit, whether it is done manually, chemically or mechanically, is to reduce crop load early in the fruit development period to maximize return of bloom (for apple) and ensure attainment of commercially acceptable fruit size (for peach, nectarine, plum, apple and table grapes). Although apples and grapes are routinely thinned in many districts, satisfactory results have not always been consistent, because sensitivity to any specific thinning agent varies from clone to clone, and climatic conditions enhance or diminish the efficiency of the chemicals. In peach and nectarines hand thinning is the only practical method that is currently employed, simply because none of the many chemicals and cultural practices employed, give consistent results. Among the many methods are: Control of flower buds initiation by gibberellic ac id (M.Sc. Thesis, A. Breuer-Mizrahi (1991), The Hebrew University of Jerusalem), flower thinners (R. Gaash, S. Lavee, A. Golan, D. Brown, "Alon Hanotea", (1969), pp. 319-325), fruitlet thinners (S. Morini, G. Vitagliano, C. Xiloyannis, I. Ant. Soc. Hort. Sci., 40, (1976), pp. 237-247, and G. E. Stembridge, C. G. Gambrell, Ibid., 96, (1971) pp. 7-9), urea, as well as cold water.
Two major problems are associated with these methods:
1. Thinning intensity was inconsistent and difficult to control. PA0 2. The broad range of effect makes it impossible to determine a precise developmental stage (of the flower or the fruitlet) for optimal treatment
In grapes, gibberellic acid is routinely used for thinning Tompson cultivar. The results, however, require always hand thinning for adjustment. The same treatment in Perlette is not practiced for the last 15 years, since it was found that in this variety, the gibberellic acid causes irregular berry size (shortberries), therefore hand thinning is practiced in Perlette.
The series of events that occur between pollination and fertilization include processes of specific recognition of pollen grains and pollen tubes by the pistil tissues. The control of the pistil on pollen germination and pollen tube growth has been mainly studied in self-incompatible systems, in which arrest of pollen tube is the consequence of a contact between pollen and pistil of the same genotype. In self-compatible matings there is some evidence that the pollen tube is directed toward the ovule by trophic and chemotropic effect in the style. However, there is still a paucity of information about the female control of the male gametophyte. Most economically important apple ((Malus domestica) cultivars require cross-pollination for fruit set. Peach (Prunus persica) is known as self-compatible. In these species the growth of pollen tubes in the style has been thoroughly investigated by physiological and histochemical studies. It has been established that the kinetics of pollen tube growth is controlled by each part of the gynoecium, starting with the stigma and ending with the ovule. However, the nature of this control, or any stylar product in which it is accomplished by, have not been characterized in these works.
Recently a major stylar protein (S-protein) was isolated from the self-incompatible species Nicotiana alata. The gene that encodes for the protein was cloned and sequenced and was found to share homology with fungal RNase. It was established that this protein is indeed RNase, and was found to arrest pollen tube growth (B. A. McClure et al., Nature, 1989, pp. 955-957). However, the RNase was also found in stylar diffusates of self-compatible species such as Nicotiana tabacum (McClure et al., 1989) and more recently in peach (Roiz and Shoseyov, 1993, unpublished data) and citrus (Roiz, Shoseyov and Goren. 1993, unpublished data). It is speculated that the RNase penetrate the pollen tube, degrade ribosomal and mRNAs and thereby cause its abortion.
Closely related to the thinning of the fruits is the reduction of seeds in citrus plants, which is very desirable from a practical as well as commercial point of view. Until now there is no known reliable method for reducing the seeds in citrus plants.
Similarly, there is no known reliable method for reducing the incidence of shotberry in grapes.
RNase is a known enzyme obtainable from a large variety of sources. One main source is the fermentation of various fungi.